Creating splines on a surface but lining them up on one axis

I have a curved surface I would like to draw a rectagularshape on but I can not get the points to line up alone the z axisso it is a straight line. How do I get all the points to be aloneone axis. I tried to do that but the relationship was not allowed.how do you take curvature out of a spline? any help would begreat.SolidworksSurfacing